Lectins are proteins which are defined by their ability to bind carbohydrates specifically and to agglutinate cells. Lectins have been shown to be involved in a wide variety of cellular functions including cell-cell and cell-matrix interactions. Lectins are widespread among plants, invertebrates and mammals.
Animal lectins have been grouped into four distinct families: 1) C-type lectins, which include selectins; 2) P-type lectins; 3) galectins (formerly termed S-type lectins or S-Lac lectins); and 4) pentraxins Barondes SH et al. (1994) J. Biol. Chem. 269:20807-10!. The C-type lectins bind carbohydrate ligands in a Ca.sup.2+ -dependent manner and are structurally related to the asialoglycoprotein receptor. Selectins, a subcategory of the C-type lectins, are composite transmembrane molecules which are involved in cell-cell interactions. The selectins include lymphocyte homing receptors and platelet/endothelial cell surface receptors Stoolman (1989) Cell 56:907-10!.
C-type animal lectins contain Ca.sup.2+ -dependent carbohydrate-recognition domains (CRDs). The prototypical C-type animal lectins are integral membrane proteins (e.g., the asialoglycoprotein receptor); however, a number of soluble C-type animal lectins have been identified. One group of soluble C-type animal lectins, termed collectins or Group III C-type lectins, comprise proteins having both lectin- (i.e., CRD) and collagenous-like domains within a single polypeptide Drickamer (1993) Curr. Opin. Struct. Biol. 3:393!. Another group of soluble C-type animal lectins, termed Group IV C-type lectins, comprise free CRDs which are not joined to other polypeptide domains (other than a signal peptide utilized in secretion) Drickamer (1993), supra!. The soluble C-type animal lectins comprising free CRDs found in mammals are most closely related to proteins identified in invertebrates and lower vertebrates (e.g., snakes).
Proteins recognized as members of the Group IV C-type lectins appear to be members of a multigene family termed the reg/PSP multigene family Drickamer (1993), supra and Unno et al. (1993) J. Biol. Chem. 268:15974!. The reg/PSP multigene family comprises genes encoding secretory proteins which are expressed in the pancreas; the ectopic expression (i.e., expression in a tissue which does not normally express reg/PSP proteins) of some members of the reg/PSP family is associated with disease states such as tumors and Alzheimer's disease.
The first member of the reg/PSP multigene family was identified in a cDNA library derived from rat regenerating pancreatic islets Terazono et al. (1988) J. Biol. Chem. 263:2111!. This gene was termed reg (regenerating gene) and is now known as the regI.alpha. gene. The regI.alpha. gene product has been called by different investigators reg protein, regI.alpha. protein, lithostathine, islet cell regeneration factor (ICRF), pancreatic stone protein (PSP) and pancreatic thread protein (PTP) Terazono et al. (1988), supra; Moriizumi et al. (1994) Biochem. Biophys. Acta 1217:199; Dusetti et al. (1993) Biochem. Biophys. Acta 1174:99; Rouquier et al. (1991) J. Biol. Chem. 266:786; and de la Monte et al. (1990) J. Clin. Invest. 86:1004!. The clear association between reg gene expression and islet cell replication in vitro has lead to the suggestion that the regI.alpha./lithostathine protein has a growth-promoting activity for islet .beta.-cells Unno et al. (1993), supra!. Human regI.alpha. mRNA is expressed in colon and rectal tumors although it is not expressed in normal colon or rectal tissue. Thus, ectopic expression of regI.alpha. protein is associated with tumorigenesis. Elevated levels of regI.alpha. protein has been found in the brains of patients suffering from Alzheimer's disease as well as in the brains of middle-aged individuals with Down's syndrome Ozturk et al. (1989) Proc. Natl. Acad. Sci. USA 86:419 and de la Monte et al. (1990) J. Clin. Invest. 86:1004!. RegI.alpha. MRNA is expressed in the developing human brain, but not in normal adult brain; expression of regI.alpha. is seen in adult brain which undergoing regenerative sprouting. Given its pattern of expression (e.g., expression in regenerating pancreatic islets and brain, expression in tumors), it appears that regI.alpha. protein is associated with cell growth.
Other members of the reg/PSP multigene family are the genes encoding pancreatitis-associated proteins (PAPs) which have been identified in humans, mice and rats Iovanna et al. (1991) J. Biol. Chem. 266:24664; Orelle et al. (1992) J. Clin. Invest. 90:2284; Itoh and Teraoka (1993) Biochem. Biophys. Acta 1172:184; and Dusetti et al. (1994) Genomics 19:108!. The reg/lithostathine and PAP proteins characterized to date share about 45-65% identity on the amino acid level.
The PAP proteins are secretory proteins which are stored in zymogen granules prior to secretion Keim et al. (1991) Gastroenterol. 100:775!; PAP is present at low levels in normal pancreas but is rapidly overexpressed during the acute phase of pancreatitis. PAP, like other members of the reg/PSP family, shares sequence similarity with the carbohydrate-binding domain of C-type lectins which likely explains the ability of PAP to induce aggregation of bacteria Iovanna et al. (1991), supra!. The ability to aggregate bacteria has lead to the suggestion that PAP is involved in the control of bacterial proliferation, a frequent complication of pancreatitis. PAP has been shown to be able to bind lactose Christa et al. (1994) FEBS Lett. 337:114!.
Three PAP genes, PAP I-III, have been identified in rats. All three PAP genes are expressed during the acute phase of pancreatitis. Rat PAP I and PAP III are expressed constitutively in the intestine and their expression is induced by feeding. Rat PAP II is not expressed in the intestine. Rat PAP I and PAP III share 66% amino acid identity; rat PAP II and PAP III share 63% amino acid identity; rat PAP I and PAP II share 58% amino acid identity. A homologue of rat PAP I has been identified in cows BPTP; de la Monte et al. (1990), supra!.
A human homolog of the rat PAP I gene, human PAP or human PAP I, has been identified Orelle et al. (1992) J. Clin. Invest. 90:2284!. The human PAP I protein is the same size as the rat PAP I protein (175 amino acids) and these two proteins share 71% amino acid identity, including conservation of 7 cystine residues. Both the rat and the human PAP I proteins are synthesized as preproteins having an N-terminal signal peptide of 26 amino acids. Expression of the human PAP I mRNA is increased in necrohemorragic pancreatitis. Serum levels of human PAP I were found to be near background levels in normal individuals; in individuals suffering from acute pancreatitis or acute exacerbations of chronic pancreatitis, human PAP I levels increased 24-140 times the background level Orelle et al. (1992), supra!. Thus, human PAP I appears to serve as a marker of acute pancreatitis.
The human PAP I gene is also referred to as the HIP gene Lasserre et al. (1992) Cancer Res. 52:5089!. The HIP gene was identified by differential screening of a human primary liver cancer (hepatocellular carcinoma) library. The human PAP I/HIP gene is not expressed in normal adult or fetal liver; expression of PAP I/HIP is limited to the pancreas and small intestine in normal tissues. Thus, the ectopic expression of PAP I/HIP is associated with tumorigenesis in the liver. In addition, PAP I/HIP MRNA is expressed in human pancreatic cell hyperplasia (hyperinsulinism) (Lasserre et al., supra).
Proteins expressed by the reg/PSP multigene family represent an important family of proteins which are involved in maintenance of proper pancreatic function as well as in the regulation of cell proliferation and/or differentiation. Discovery of new molecules related to or in the mammalian reg/PSP multigene family is useful for the development of new diagnostic or therapeutic compositions.